fix incorrect file state in Attachment#restore

test plan:
 0. BEFORE APPLYING THIS PATCHSET,
    a) delete a file in a course
    b) restore it via /courses/X/undelete
    c) notice that the restored file is not visible
       to students (but is visible to teachers)
 1. apply this patchset and run migrations
 2. confirm that the file deleted in step 0 is now visible
    to students
 3. delete and restore another file
 4. confirm that the restored file is visible to students

fixes CNVS-15616

Change-Id: Id8be88e63686cd0811409dfe6adb8bbf1a9d74c3
Reviewed-on: https://gerrit.instructure.com/41286
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>